The present invention relates to an optical transmission system and a method of controlling switching of a transmission line in the event of a transmission line failure. More particularly, the invention relates to an optical transmission system, which has N-number of currently working optical transmission lines and one protection (standby) optical transmission line, in which data is sent and received between stations via the transmission lines in a frame format having overhead, and to a 1:N transmission line switching control method in such a system.
Shelf
Several basic shelves are prepared, the selves are assembled to construct a terminal station, a repeater station and a signal regenerator, and these are used to construct an optical transmission system.
FIGS. 34A and 34B are diagrams illustrating an HS (high-speed) shelf, and FIGS. 35A and 35B are diagrams illustrating a TRIB (tributary) shelf. As shown in FIG. 34A, an HS shelf 150 has line-optical interfaces 151, 152 for interfacing OC-48 (2.4 Ghz) optical transmission lines, a switch 153, an alarm interface 154, a controller 155, a clock source 156, a tributary-side optical interface 157, and a tributary-side electrical interface 158. As shown in FIG. 34B, the line-optical interfaces 151, 152 respectively have O/E converters 151a, 152a, E/O converters 151b, 152b for converting electric signals to optical signals, demultiplexers (DMUX) 151c, 152c for demultiplexing a higher-order group signal (an OC-48 optical signal) into three types of signals STS-1, STS-3C, STS-12c, and multiplexers 151d, 152d for multiplexing the signals STS-1, STS-3C, STS-12c. The switch 153 has a function for passing the three types of signals demultiplexed by the demultiplexers 151c, 152c or for dropping these signals on the tributary side. Further, the switch 153 switches the signals STS-1, STS-3C, STS-12c, which have been inserted from the tributary side, to the E (East) or W (West) direction.
As shown in FIG. 35A, an HS shelf 160 has interfaces 161, 162 for lower-order group signals (DS 3.times.12ch, STS-1.times.12ch, OC-3/3c.times.2ch, OC-12/12C.times.1ch), a switch 163 and an interface 164 for interfacing the HS shelf. As shown in FIG. 35B, the tributary-side interfaces 161, 162 respectively have multiplexer/demultiplexers (MUX/DMUX) 161a, 162a for multiplexing the lower-order group signals to signals STS-1, STS-3C, STS-12c, entering these signals into the switch 163, demultiplexing signals that have entered from the switch 163 and then outputting these signals, and interfaces 161b, 162b for interfacing multiplexers located within station.
LTE, LNR ADM, REG
By combining the HS shelf 150 and TRIB shelf 160, it is possible to construct an LTE (line terminal equipment), which serves as a terminal station for an optical transmission line, as shown in FIGS. 36A, 36B, or an LNR ADM (linear add/drop multiplexer), which serves as a repeater station (D/I: drop/insert), as shown in FIG. 36C. In a case where the system is expanded, it is required that the terminal station of LTE construction be changed to a repeater station. In such case the two LTEs of FIGS. 36A and 36B are connected back to back, as depicted in FIG. 37, thereby forming a repeater station provided with a function equivalent to that of the LNR ADM having the add/drop function. Furthermore, a signal regenerator REG (regenerator) can be constructed by allowing signals to pass through using the switch of the HS shelf 150. In the LTEs illustrated in FIGS. 36A, 36B, only the line-optical interface on one side of the HS shelf is used.
Construction of Transmission System
A point-to-point optical transmission system can be constructed by using the LTEs, arranged as set forth above, as terminal stations (station A and station B) of an OS-48 optical transmission line, as illustrated in FIG. 38. Further, a ring system can be constructed by connecting LNR ADMs in a ring-shaped configuration, as shown in FIG. 39. Furthermore, a linear ADM system can be constructed by using the LTEs as terminal stations (stations A and C) and using the LNR ADM as a repeater (station B), as depicted in FIG. 40.
Further, as shown in FIG. 41, a 1+1 line switching point-to-point system can be constructed by adding 2.4 G switch controllers (2.4 G SW CONT) onto a 1+1 arrangement having one working line WORK1 and one protection (standby) line PTCT. Similarly, as shown in FIG. 42, a 1:N line switching point-to-point system can be constructed by adding 2.4 G switch controllers (2.4 G SW CONT) onto a 1:N arrangement having N-number of working lines WORK1.about.WORKN and one protection line PTCT.
Lower-order group signals are capable of being supported with regard to the LTE in the protection line as well. The function in which the LTE of the protection line supports a lower-order group signal is referred to as PCA (protection channel access). In an ordinary system, optical transmission lines can be utilized efficiently by transmitting a PCA signal using a protection line, namely a line which is not transmitting a signal. It should be noted that when a working line is switched over to the protection line, transmission of the PCA signal using the protection line is no longer carried out.
Changeover of Optical Signal Line
In the 1:N line switching point-to-point system, the sending and receiving of information relating to switching of the OC-48 optical signal line is performed using K1/K2 bytes of overhead bytes stipulated by a SONET (synchronous optical network) standard, which is in line with the new synchronous network standard of North America.
(1) Frame format
FIG. 43A is a diagram for describing the frame format of a SONET STS-3(OC-3). One frame consists of 9.times.270 bytes. The first 9.times.9 bytes constitute section overhead (SOH), and the remaining bytes constitute path overhead (POH) and payload (PL). Section overhead is for transmitting information (a frame synchronizing signal) representing the beginning of the frame, information specific to the transmission line (information for checking error at the time of transmission, information for maintaining the network, etc.), and a pointer indicating the position of the path overhead POH. Further, the path overhead POH is for transmitting information for end-to-end monitoring within the network, and the payload PL is for transmitting 150 Mbps information.
The section overhead SOH is composed of repeater section overhead of 3.times.9 bytes, a pointer of 1.times.9 bytes and multiplex section overhead of 5.times.9 bytes. The repeater section overhead has bytes A1.about.A2, C1, B1, E1, F1 and D1.about.D3, as shown in FIG. 43B. The multiplexer section overhead has bytes B2, K1.about.K2, D4.about.D12 and Z1.about.Z2. The repeater section overhead and multiplexer section overhead have a number of undefined bytes and use thereof is left to the communications manufacturer.
FIG. 44 is a diagram for describing the SONET OC-12 frame format produced by multiplexing SONET OC-3 frames. The frame is composed of section overhead SOH of 9.times.9.times.4 bytes, path overhead POH of 9.times.4 bytes and a payload PL of 9.times.260.times.4 bytes. A SONET OC-48 frame is similarly constructed.
Among the overhead bytes, the K1 byte is used mainly to request switching and designates the level of the switching request and the line switched. The K2 byte is used mainly to respond to the K1 byte. In addition, this is used to express the system architecture, the switching mode and AIS/FERF (AIS: alarm indication signal; FERF: far end receive failure). Switching requests include, in addition to the switching request at the time of signal failure, switching requests based upon lock-out, a forced switch and a manual switch. FIGS. 45 and 46 illustrate the arrangement of the K1/K2 bytes stipulated by the SONET standard, as well as a list of the meanings of the bytes.
(2) K1 byte
The first four bits b1.about.b4 of the K1 byte represent a switching request, and the last four bits b5.about.b8 represent a switching line. A maximum of 14 transmission lines can be designated. "Lockout of Projection" is a switching request which inhibits switching to a protection transmission line. "Forced Switch" is an artificial switching request for a designated transmission line. If a changeover has been made, no changeover is made to any other line that has failed. "SF" (signal failure) is a switching request for when a signal on a transmission line has been lost. This request has two priorities, namely high and low. "SD" (signal degrade) is a switching request in response to deterioration of a signal on a transmission line and has the two priorities high and low. It should be noted that the SF switching request has a higher priority than the SD switching request. "Manual Switch" is an artificial switching request. When a failure has occurred elsewhere, priority is given to changeover of this switch. "Wait-to-Restore" is a request which, if a request for switching back has been issued after restoration of a failed line, performs switch-back upon elapse of a prescribed period of time. "Exercise" is a request for automatically diagnosing, by actually performing switching, whether the switching operation has been performed normally. "No Request" is sent when operation is normal or when a bridge is removed.
Switch-back modes that can be set are of two types, namely a non-revertive mode in which, if a fault that caused switching has been eliminated, the line to which the changeover has been made is kept as is and is not switched back, and a revertive mode in which the line to which the changeover has been made is switched back to the original line. The former is used mainly in case of the 1+1 arrangement and the latter in case of the 1:N arrangement. The revertive mode has the WTR (wait to restore) function. Specifically, after the cause of switching is eliminated, switch-back is performed not immediately but upon elapse of a specific period of time. This is a function which prevents noisy switching and is stipulated as being between 5 and 12 minutes according to the SONET standard.
In a case where there is contention for a protection line at the time of switching, priority is given to the switching request having the higher level. Further, two degrees of priority (LOW/HIGH) can be set for each line. In a case where switching requests have the same level, the line having the higher degree of priority is switched. In a case where switching requests have the same level and the degrees of priority of the lines are also the same, the line that issued the switching request first is switched. In a case where the levels of the switching requests, the degrees of priority of the lines and the timings at which the switching requests were issued are the same, the line having the youngest line number is given precedence in changeover. This is the order of priority stipulated by the SONET standard. However, there are cases where other orders of priority are requested depending upon the customer.
(3) K2 byte
Bits b1.about.b4 of the K2 byte designate the number of the transmission line. These bits are nulled (0000) in a case where the bits b5.about.b8 of the received K1 byte are null and become the number of the transmission line switched in other cases. The b5 bit indicates the network construction; a "1" indicates the 1+1 system and a "0" indicates the 1:N system. The bits b6.about.b8 indicate the particular switching mode, the details of a failure, etc. There are two types of switching modes, namely a unidirectional mode in which only a unidirectional signal is switched, and a bidirectional mode in which signals in two directions are switched simultaneously.
(4) Switching sequence using K1, K2 bytes
In the unidirectional mode, the B station sends the K1 byte (switch request) to the A station upon detecting SF (signal failure), as shown in FIG. 47A. The A station performs bridge control with respect to the line designated by the K1 byte (switch request) received. Bridge control refers to control for sending identical signals to both the working line and the protection line. After performing bridge control, the A station sends the B station the K2 byte (switch response) corresponding to the K1 byte received. Upon receiving the K2 byte, the B station performs switch control. Switch control means control in which the line signal of the designated reception direction is changed over to the protection line.
In the bidirectional mode, the B station sends the K1 byte (switch request) to the A station upon detecting SF, as shown in FIG. 47B. The A station performs bridge control with respect to the line designated by the K1 byte (switch request) received, sends back the K2 byte (switch response) in the same manner as in the unidirectional mode, and simultaneously sends a K1 byte designating a reverse request (RR). Upon receiving the RR, the B station performs switch control and bridge control with respect to the line designated by the K1 byte which it itself sent, and sends the K2 byte (switch response) to the A station. Upon receiving the K2 byte (switch response), the A station performs switch control.
The following problems arise in the currently existing SONET standard.
(1-1) A PCA signal passed utilizing the protection line when a changeover has not been made is interrupted at execution of "exercise" (self-diagnosis of the switching operation) in the working line.
(1-2) According to the SONET standard, switching priority based upon the importance of the failure is set to be higher than switching priority based upon the importance of the line. However, there are also users who desire that the switching priority based upon the importance of the line be set to be the higher, in which case the SONET standard cannot be accommodated.
(1-3) The number of working lines per system is limited to 14 or less by the existing SONET standard, and a 1:N system having 15 or more working lines cannot be constructed.
(1-4) The existing SONET standard stipulates only a 1:N line switching point-to-point system; there are no standards for a 1:N line switching LNR ADM system or 1:N line switching nested system. In the 1:N line switching point-to-point system, the switching section is a single section from one terminal station to another. More specifically, the SONET standard only supports line switching of one section. There is no standard for a case where a transmission line between terminal stations is divided into a plurality of sections and line switching is performed per section.
(1-5) In a case where a transmission line between terminal stations is divided into a plurality of sections and line switching is performed per section, LNR ADMs (repeater stations) are placed at the boundaries of the switching sections. When a K1/K2 byte is entered in such a case, control is required for either accepting and terminating a byte or not accepting the byte and passing it through to the next stage. Further, when a K1/K2 byte has been sent to the next stage erroneously, line switching takes place in the wrong section.
(1-6) The SONET standard stipulates an ambiguous 5.about.12 min as the time for WTR (wait to restore). A specifically set time or interval is not clearly indicated.
Further, the following problems arise in the conventional optical transmission system:
(2-1) Each terminal station in the system is managed and controlled using a craft interface via TL-1 message and a 1:N switching function unit is managed and controlled using a separately provided independent craft interface. Operation must be performed using different procedures and different operating systems.
(2-2) In a case where a repeater (D/I) station is constructed using LTEs instead of an LNR ADM, a thru-signal also is connected back to back in a lower-order group. The problem which arises in a large number of shelves in the station.
(2-3) In a case where a D/I station is constructed using LTEs, the connection is back to back (the DS 3 signal in the prior art). Consequently, even a signal that is to be passed through is temporarily terminated and the line section becomes one which is different from the line section stipulated by the SONET standard.
(2-4) In the multiplexer (MUX) of protection shelves when switching is performed, the presently prevailing input signal can be processed only in units (STS1C, 3C or 12C) set for each shelf. Consequently, in a case where lower-order group signals of each working line are constituted by units of various types and a signal constituted by a unit other than the set units is switched, the changeover cannot be performed while satisfying the SONET standard. In other words, if working line 1 is set by OC-3C, working line 2 by STS-1 and the protection shelf by STS- 1, then OC-3C is broken down to the STS-1 level when working line 1 is switched.
(2-5) In order to change the 1:N point-to-point system to a 1:N nested switching system or vice-versa, it is necessary to replace units for monitoring/control.
(2-6) In an existing system, an LTE must be used in the protection line of the terminal station. Consequently, in a case where the station has become a repeater owing system enlargement, it becomes necessary to change the LTE (i.e., to enlarge the shelf).
(2-7) In a 1:N nested system, the PCA channel cannot be used between repeater stations; it can be used only between terminal stations.